Field of the Invention
This disclosure relates to wireless communications and in particular to enhancements to controlling interference using almost blank subframes (ABS).
Description of the Related Art
Mobile traffic has grown at a very fast pace and the trend is continuing. To meet the mobile traffic growth demand and improve the end user experience, mobile service providers are actively looking for mechanisms to improve system capacity and end user experience. Deploying small cells (such as metro cells or pico cells) can help to improve coverage and capacity. Metro cells can be deployed in many different ways. Many mobile operators deploy metro cells co-channel with macro cells for use in spectrum limited situations. Such deployments result in heterogeneous networks (HetNets) that include different kind of access points serving different kinds of cells with overlapping coverage areas. For example, HetNets may include macro cells and small cells (metro cells, femtocells, and/or pico cells) with overlapping coverage areas. Communication devices within the overlapping coverage areas can suffer from significant co-channel interference.
To mitigate interference in a HetNet environment, Enhanced Inter-Cell Interference Coordination (eICIC) mechanisms can be used, in which the macro cell avoids scheduling data in “protected” sub-frames as specified by Third Generation Partnership Project (3GPP) Release 10 (Rel-10). The protected sub-frames of eICIC are known as “Almost blank subframes” (ABS) and do not transmit any traffic channels and mostly comprise low-power control channel frames. User equipment (UE) located in the pico/metro cell range expansion area is only scheduled on the protected subframes. That allows the pico/metro cell to communicate with the UE with reduced interference from the macro cell. For the inner part of the metro cell, user equipment can be scheduled on any subframe (almost blank or otherwise) as the transmit power of the metro cell is strong enough to overcome interference. For the user equipment served by the macro cell, the UE can only be scheduled during non-protected subframes.
A further enhancement, Further Enhanced Inter-cell Interference Coordination (FeICIC) was proposed in 3GPP Release 11 in which inter-cell interference can be further reduced through cancellation of interference on common control channels of ABS caused by Common Reference Signals (CRS) of high power macro cells and critical broadcast system information can be better detected in the cell range extension region around the metro cell. Also, to relieve some negative impact on macro layer capacity in eICIC/FeICIC, reduced power ABS was proposed to improve HetNet capacity (instead of zero-power ABS) by reducing capacity loss in macro cells.
However, further improvement to ABS approaches is desirable to avoid adverse impacts on macro cell capacity and improve overall throughput.